Finite element analysis of 4ENF testing of stitched carbon/epoxy laminate

Stitching technique improves both mode I and mode II delamination toughness significantly by reducing the opening and sliding displacement, respectively. However, measuring mode II delamination behaviour of stitched composites using three point end notched flexure test (ENF) is facing a serious problem where the premature failure of specimen occurs prior to delamination propagation. This work is addressed to conduct numerical study of four points end notched flexure (4ENF) test of stitched composites. Four models were simulated including unstitched unidirectional, unstitched multidirectional laminate, stitched multidirectional laminate with and without tabs on top and bottom of specimen. Furthermore, stitch threads were modelled using spring connectors with user define force-displacement relationship. Load-displacement and crack propagation length vs. displacement curves were plotted. Stress distributions across the thickness were also presented to recognize the possibility of specimen damage during the test. The results showed that simple 4ENF test seems to be quite enough for evaluating mode II critical energy release rate (GIIC) at the initial crack length of 35mm. However to evaluate R-curves (GII vs. crack propagation length) of stitched multidirectional laminate, tabbed 4ENF test specimen is recommended.